The invention relates generally to the area of DC servomotor control circuits, and specifically the invention provides a circuit for controlling the motor current as a function of a speed error signal.
In DC servomotor circuits, as are used for example in numerically controlled machine tools, it is known that the motor current must be limited in order to avoid damaging effects. If too high a current is commutated, the life of the commutator will be shortened; and the value of the current which can safely be commutated falls with increasing motor speed. Furthermore, permanent magnet DC motors are subject to an overriding current limit above which irreversible partial demagnetization of the field magnets occurs leading to a loss of torque.
Although not limited specifically to such a situation, the invention will largely be described in the context of energization by controlled rectified AC wherein mean currents are increased and decreased by respectively lengthening and shortening the proportion of a half cycle over which conduction occurs. The invention is equally applicable to other ways of controlling the energization of a DC motor. In controlled rectifier systems, the phase angle at which conduction commences may be referred to as the firing angle. Advancing and retarding this angle respectively increases and decreases the energization. The invention applies equally to the conditions of acceleration and deceleration of a maximum load at any speed.
A DC servomotor circuit may include a speed loop with a comparator which compares a speed command signal with an actual speed command signal to produce a speed error signal. This error signal is used to directly control the firing angle. Alternatively, the speed loop may be followed by a current loop with a second comparator which compares the speed error (acting as a current command signal) with an actual current signal to produce a current error signal. This signal then controls the firing angle.
It is known to use a parallel acting limit circuit which limits the speed error signal and retards the firing angle when the current exceeds a set limit. The main problem with this type of circuit is that when the motor is accelerating the current limit is always temporarily exceeded while the limit circuit is responding to and counteracting this situation.
We have previously developed a system of a current limiting servomotor circuit with a speed loop and a current loop. The speed error signal is subject to a limit which has a first fixed level up to a certain speed and thereafter has a second and smaller fixed level. The provision of two fixed limiting levels corresponding to two speed ranges gives improved current limitation. It satisfies reasonably well the requirement for a lower limit at higher speeds because of the limit on which current can be commutated. Also, because the limit is imposed on the current which can be commanded within each of the two speed ranges, the current limit is not influenced by speed. This relies upon the use of current loop as well as a speed loop. In practice, when energizing the motor by controlled rectified alternating current, a current loop creates severe design problems because the command signal (speed error signal) is a smooth slowly varying signal whereas the actual current signal, as sensed by current shunt in the motor circuit, is discontinuous and must be smoothed by an RC filter network. In a single phase system, the response to a command signal may be delayed by up to 10 milliseconds. The design of a filter network which provides the necessary smoothing but does not introduce further delays into the system is extremely difficult.
The invention provides a DC motor control circuit which is free of the above disadvantages. The drive circuit is arranged to control the energization of the motor as a function of a speed error signal thereby maintaining the motor speed at the speed commanded by the speed command signal. A limiting device is responsive to the actual motor speed to limit the level of the speed error signal according to different functions depending on the speed range of the motor.
In the prior art devices, a constant limit was used in each of the speed ranges. However, a constant limit on firing angle gives a falling limit on current. Accordingly, in the present invention, the limit in the various speed ranges increases or decreases according to the motor requirements. The limiting level is rendered dependent on measured speed, and the way in which the limit is imposed is dependent upon the details of a particular servomotor circuit. The limit may be bipolar or unipolar. The unipolar limit function or each bipolar limit function can be built up as a function of the speed signal from linear segments with slopes and break points determined in the known manner. Slopes in general are determined by gain factors; and break points may be determined by amplifier cutoffs, diode action or saturation of transistors. The limit may be imposed on the speed error signal by techniques known per se involving diodes or switching transistors.